1. Field of the Invention
The present invention relates to a digital plotting system and more particularly to a digital plotting system for displaying character information.
2. Description of the Prior Art
Many attempts have been made to inexpensively and effectively transmit and display graphic type information. One well known method for doing this is with facsimile television. In a normal TV display of 525 lines this represents a transmission of about 300,000 points of data for black and white presentation. There are several ways in which television signals may be transmitted. These include microwave, telephone or teletype transmission. Microwave type transmission can very rapidly transmit 300,000 points or bits of information. However, microwave transmission requires expensive width bandwidth (for example, 6 megacycles) equipment and involves either coaxial transmission lines or repeater stations. In addition, microwave reception is not available throughout the world as may be required in many remote locations. Therefore, facsimile graphic representation by conventional TV is not suitable where cost and worldwide availability are important factors.
It has been proposed that narrowband transmission of a binary representation of each point of each raster line may be an effective mode of transmitting graphic information. While graphic information may be transmitted by this binary representation (for example, with a binary "1" representing each dark point on the graph) it requires the narrowband transmission of approximately 300,000 points. With narrowband transmission, teletype speed being about 3,000 bits/minute, this would require about 100 minutes to transmit a single graph on a 525.times.525 display. For many applications this time requirement would be excessive.
While telephone lines are available they are expensive and have the limitation of being restricted to the land masses of the world. In addition, they involve a relatively narrow bandwidth of about 3 KC. This limits the transmission to about 2,400 bits/sec and requires about 2 minutes to transmit a 300,000 bit message. For many applications this time requirement, especially due to the high cost, would be excessive.
The present invention overcomes these disadvantages by providing a digital plotting system wherein only character information is transmitted. This results in a very large reduction in the number of data points for an average character transmission with a commensurate reduction in transmission time as compared to facsimile transmission.